[0001] This application claims the benefit of Korean Patent Application No. 10-2003-80394
filed November 14, 2003, the entire contents of which are herein fully incorporated
by reference.
[0002] The present invention relates to a method for controlling a tilt servo in an optical
disc device, and more particularly to an optical disc device and a method for correcting
a tilt of an optical disc using a focus error signal and a focus drive signal.
[0003] Recently, optical discs capable of storing digital video and audio data, for example,
digital versatile discs (DVDs) or compact discs (CDs), are widely supplied. Moreover,
optical disc devices equipped with an optical pickup for reproducing or recording
data on optical discs have been commercialized.
[0004] An optical pickup provided in an optical disc device comprises an objective lens
11, a beam splitter 12, a photo detector (PD) 13, a laser diode (LD), as shown in
FIG. 1.
[0005] In a process for manufacturing the optical disc device, a base on which the optical
pickup is mounted is finely adjusted so that the objective lens 11 of the optical
pickup is parallel with a skew-free optical disc 10 loaded into the optical disc device.
This is to prevent playback or recording performance from being degraded when the
objective lens 11 and the optical disc 10 are not parallel to each other. Thus, the
base is finely adjusted in tangential and radial directions on the basis of the skew-free
optical disc 10. This fine adjustment is referred to as the tilt or skew adjustment.
[0006] As shown in FIG. 2, the optical disc device of FIG. 1 also comprises an LD driver
15, a focus error (F.E.) detector 16, a microcomputer (Micom) 17, a focus drive 18,
a servo 19, a memory 20, etc.
[0007] The F. E. detector 16 carries out a mathematical operation on electrical signals
outputted from the photo detector 13 included in the optical pickup, for example,
a 4-element photo detector divided into A, B, C and D areas, and then detects a focus
error signal (F.E. = (A + C) - (B + D)).
[0008] In order to bring the value of the focus error signal down to zero, the microcomputer
17 generates and outputs a focus control signal based on the focus error signal. The
servo 19 generates a focus drive signal corresponding to the focus control signal
and then outputs the generated focus drive signal to the focus drive 18.
[0009] The focus drive 18 applies, to a focus actuator coil (not shown) of the optical pickup,
focus drive current corresponding to the focus drive signal. Through this operation,
a focus servo operation is performed to vertically shift the objective lens 11 so
that a detected focus error can be reduced.
[0010] Meanwhile, where a skew is present in the optical disc inserted into the optical
disc device, for example, where the optical disc is bent in the outward direction
(or the skew is present in the radial direction), the optical disc device operates
in a state in which the optical disc and the objective lens of the optical pickup
are not positioned parallel to each other while being at a tilt. Thus, the performance
of the focus servo or a tracking servo is degraded and hence the performance of a
playback or recording operation is degraded. This phenomenon is called a tilt error.
[0011] In order for the tilt error to be corrected, jitter characteristics of a radio frequency
(RF) outputted from a photo detector (PD) must be measured to calculate a tilt degree
between the optical disc and the objective lens of the optical pickup. Alternatively,
the tilt degree must be measured by a special tilt sensor. That is, an expensive jitter
measurement circuit or a tilt sensor occupying a large space is required and used
to correct the skew of the optical disc. However, this increases the size and price
of the optical disc device, which is a problem.
[0012] Therefore, the present invention has been made in view of the above and other problems,
and it is one object of the present invention to provide an improved method for correcting
a tilt error in an optical disc device.
[0013] It is another object of the present invention to provide an optical disc device and
a method of correcting a tilt error in an optical disc device, which overcome the
problems and limitations associated with the related art.
[0014] It is another object of the present invention to provide a method capable of easily
measuring a tilt degree between an optical disc inserted into an optical disc device
and an objective lens.
[0015] In accordance with an aspect of the present invention, the above and other objects
can be accomplished by the provision of a method for controlling a tilt servo in an
optical disc device, comprising the steps of: (a) detecting a specific signal associated
with a focus servo; and (b) generating a tilt servo control signal for an objective
lens according to a variation amount of the specific signal.
[0016] According to an aspect of the present invention, the specific signal is a focus error
signal or a focus drive signal, and an offset signal based on the focus error signal
or the focus drive signal is used. The specific signal may be detected in predetermined
section units, and the variation amount of the specific signal is calculated. The
tilt servo control signal corresponding to the variation amount of the specific signal
may be stored in a memory in advance.
[0017] These and other objects of the present application will become more readily apparent
from the detailed description given hereinafter. However, it should be understood
that the detailed description and specific examples, while indicating preferred embodiments
of the invention, are given by way of illustration only, since various changes and
modifications within the spirit and scope of the invention will become apparent to
those skilled in the art from this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The above and other objects, features and other advantages of the present invention
will be more clearly understood from the following detailed description taken in conjunction
with the accompanying drawings, in which:
[0019] FIG. 1 is an explanatory view illustrating an example of physically adjusting a tilt
of an optical pickup in a process for manufacturing an optical disc device according
to a related art;
[0020] FIG. 2 is a block diagram illustrating a configuration of the optical disc device
of FIG. 1;
[0021] FIG. 3 is a block diagram illustrating the configuration of an optical disc device
to which a method for controlling a tilt servo is applied in accordance with one embodiment
of the present invention;
[0022] FIG. 4 is a flowchart illustrating the method for controlling the tilt servo in the
optical disc device in accordance with the present invention;
[0023] FIG. 5 is a block diagram illustrating the configuration of an optical disc device
to which a method for controlling a tilt servo in the optical disc device is applied
in accordance with another embodiment of the present invention; and
[0024] FIG. 6 is an explanatory view illustrating a relationship between optical disc bending
and a focus offset causing a tilt error according to another embodiment of the present
invention.
DETAILED DESCRIPTION OF PREFFERRED EMBODIMENTS
[0025] Now, an optical disc device and a method for controlling a tilt servo in the optical
disc device in accordance with preferred embodiments of the present invention will
be described in detail with reference to the annexed drawings.
[0026] FIG. 3 is a block diagram illustrating the configuration of an optical disc device
to which a method for controlling a tilt servo is applied in accordance with one embodiment
of the present invention. Referring to FIG. 3, the optical disc device comprises an
optical pickup including an objective lens 11, a beam splitter 12, a photo detector
(PD) 13, a laser diode (LD), etc. Further, the optical disc device comprises an LD
driver 15, a focus error (F.E.) detector 16, a microcomputer (Micom) 117, a focus
drive 18, a servo 119, a memory 20, a low pass filter (LPF) 21 and a tilt drive 22.
All the components of the optical disc device are operatively coupled. The focus servo
operation in FIG. 1 is also performed in the device of FIG. 3 and thus the description
thereof is omitted.
[0027] Only a low frequency component of a focus error signal outputted from the F. E. detector
16 is inputted to the microcomputer 117 through the LPF 21. The microcomputer 117
then generates and outputs a tilt control signal for correcting a tilt error of the
objective lens 11 according to a value of the low frequency component of the focus
error signal outputted from the LPF 21.
[0028] The servo 119 generates a tilt drive signal corresponding to the tilt control signal
from the microcomputer 117 and then outputs the generated tilt drive signal to the
tilt drive 22. The tilt drive 22 applies, to a tilt actuator coil provided in the
optical pickup, the tilt drive current corresponding to the tilt drive signal, such
that a tilt servo operation is performed for the objective lens 11. This operation
will now be described in more detail referring to FIG. 4.
[0029] FIG. 4 is a flowchart illustrating the method for controlling the tilt servo of the
optical disc device in accordance with the present invention. Although the method
of FIG. 4 is applicable to the optical disc device of FIG. 3, it is not limited to
it and can be applied to other suitable devices/systems.
[0030] When a typical focus servo operation is performed in the optical disc device (S10),
the F.E. detector 16 outputs a focus error signal based on a focus error detected
thereby, and the LPF 21 filters the focus error signal and then outputs the filtered
focus error signal to the microcomputer 117. The filtered focus error signal has only
a low frequency component.
[0031] The microcomputer 117 detects a value of the low frequency component of the focus
error signal outputted from the LPF 21 (S11). The microcomputer 117 detects the low
frequency component value of the focus error signal for a predetermined period because
the focus error signal is not constant due to random noise, characteristics of focus
feedback servo, etc. (S12).
[0032] When the predetermined period has passed (S12), the microcomputer 117 accumulates
or integrates all the low frequency component values detected during the predetermined
period to calculate a digital integration value or a mean value between digital integration
values (S13).
[0033] Subsequently, the microcomputer 117 compares the calculated digital integration value
or the mean value with reference values within a tolerant range of a tilt error. The
reference values are preset values obtained based on a result of experimentation in
a process for manufacturing the optical disc device. The reference values can be stored
in the memory 20.
[0034] If it is determined that the calculated digital integration value or the calculated
mean value exceeds or falls outside the reference range as a result of the comparison
(S14), the microcomputer 117 generates and outputs a tilt control signal corresponding
to the digital integration value or the mean value. The servo 119 generates a tilt
drive signal corresponding to the tilt control signal output from the microcomputer
117 and outputs the generated tilt drive signal to the tilt drive 22. The tilt drive
22 applies, to the tilt actuator coil provided in the optical pickup, tilt drive current
corresponding to the tilt drive signal to move the objective lens accordingly (S15).
Through this operation, a tilt servo operation is performed to vertically shift inward
and outward sides of the objective lens 11 in directions opposite to each other, respectively.
[0035] This tilt servo operation is continuously repeated until a recording or playback
operation of the optical disc device is terminated (S16). Only after a value of the
low frequency component of the focus error signal filtered by the LPF 21 exceeds the
preset reference range, the microcomputer 117 may enable the tilt servo operation
for correcting the tilt error to be immediately performed according to the present
invention.
[0036] FIG. 5 is a block diagram illustrating the configuration of an optical disc device
to which a method for controlling a tilt servo in the optical disc device is applied
in accordance with another embodiment of the present invention. The optical disc device
shown in FIG. 5 is similar to that shown in FIG. 3. However, the optical disc device
shown in FIG. 5 filters using a LPF 221 a focus drive signal detected and output from
a servo 219 and then outputs the filtered focus drive signal to a microcomputer 217.
This differs from the optical disc device shown in FIG. 3 filtering a focus error
signal detected from the F.E. detector 16. The microcomputer 217 receives the low
frequency components of the focus drive signal for a predetermined time period and
generates a tilt control signal based on the low frequency components. The servo 219
receives the tilt control signal and then generates and outputs a tilt drive signal
accordingly to the tilt drive 22, which in turn moves the objective lens 11. In other
words, in this embodiment, the method steps of FIG. 4 are performed using the low
frequency components of a focus drive signal (see S11 in FIG. 4), and not based on
a focus error signal as in the device of FIG. 3.
[0037] In accordance with another embodiment of the present invention, a tilt servo operation
is performed on the basis of a focus offset variation. As shown in FIG. 6, an optical
disc is divided into a plurality of sections in an outward direction. A focus offset
measured in each section of the disc is compared with that measured in its neighbor
section, such that the focus offset variation is calculated. A tilt control signal
is then generated on the basis of the calculated focus offset variation, such that
the tilt servo operation can be performed to drive a tilt actuator in response to
the generated tilt control signal. This method can be implemented in the optical disc
device of FIG. 3 modified to detect the focus offset variation.
[0038] For reference, the focus offset is a value necessary for driving the actuator of
the objective lens so that the detected focus error signal is brought to zero. Furthermore,
the focus offset corresponds to a direct current (DC) or low frequency value of the
focus error or drive signal. As shown in FIG. 6, because a predetermined constant
distance (D) is maintained between the objective lens and a recording surface of the
optical disc by the focus drive signal even if the optical disc is bent or deformed,
the focus offset value reflects a form/bent shape or degree of the optical disc and
more particularly a form in which the optical disc is generally bent from the inner
circumference to the outer circumference.
[0039] As apparent from the above-described present invention, a tilt degree can be easily
measured without using expensive tilt measurement equipment or an expensive correction
circuit.
[0040] Furthermore, a tilt error is corrected by a simple method, such that performance
of a focus servo or a tracking servo can be improved greatly and thus performance
of a playback or recording operation for an optical disc can be improved greatly.
[0041] Although the preferred embodiments of the present invention have been disclosed for
illustrative purposes, those skilled in the art will appreciate that various modifications,
additions and substitutions are possible, without departing from the scope and spirit
of the invention as disclosed in the accompanying claims.
1. A method for controlling a tilt servo in an optical disc device, comprising the steps
of:
(a) detecting a specific signal associated with a focus servo; and
(b) generating a tilt servo control signal for an objective lens according to a variation
amount of the specific signal.
2. The method of claim 1, wherein the specific signal is a focus error signal.
3. The method of claim 2, wherein the specific signal is an offset signal based on the
focus error signal.
4. The method of claim 1, wherein the specific signal is a focus drive signal.
5. The method of claim 4, wherein the specific signal is an offset signal based on the
focus drive signal.
6. The method of any of claims 1 to 5, wherein the step (a) comprises the step of:
detecting a low frequency component of the specific signal.
7. The method of any of claims 1 to 6, wherein the variation amount of the specific signal
is detected in a predetermined section unit.
8. The method of claim 7, wherein the variation amount of the specific signal is calculated
by a difference between values detected in sections that neighbor with each other.
9. The method of any of claims 1 to 8, wherein the tilt servo control signal corresponding
to the variation amount of the specific signal is measured and stored in a memory
in advance.
10. The method of any of claims 1 to 9, wherein the step (b) comprises the step of:
detecting low frequency component values of the specific signal for a predetermined
period.
11. The method of claim 10, wherein the step (b) further comprises the steps of:
calculating an integrated value of the detected low frequency component values; and
generating the tilt servo control signal for the objective lens if the integrated
value falls outside a preset reference range.
12. The method of claim 10, wherein the step (b) further comprises the steps of:
calculating a mean value of the detected low frequency component values; and
generating the tilt servo control signal for the objective lens if the mean value
falls outside a preset reference range.
13. An optical disc device for controlling a tilt servo, comprising:
first means for detecting a specific signal associated with a focus servo; and
second means for generating a tilt servo control signal for an objective lens according
to a variation amount of the specific signal.
14. The optical disc device of claim 13, wherein the specific signal is a focus error
signal.
15. The optical disc device of claim 14, wherein the specific signal is an offset signal
based on the focus error signal.
16. The optical disc device of claim 13, wherein the specific signal is a focus drive
signal.
17. The optical disc device of claim 16, wherein the specific signal is an offset signal
based on the focus drive signal.
18. The optical disc device of any of claims 13 to 17, wherein the first means detects
a low frequency component of the specific signal.
19. The optical disc device of any of claims 13 to 18, wherein the variation amount of
the specific signal is detected in a predetermined section unit.
20. The optical disc device of claim 19, wherein the variation amount of the specific
signal is calculated by a difference between values detected in sections that neighbor
with each other.
21. The optical disc device of any of claims 13 to 20, further comprising:
a memory wherein the tilt servo control signal corresponding to the variation amount
of the specific signal is measured and stored in the memory in advance.
22. The optical disc device of any of claims 13 to 21, wherein the second means detects
low frequency component values of the specific signal for a predetermined period.
23. The optical disc device of claim 22, wherein the second means calculates an integrated
value of the detected low frequency component values, and generates the tilt servo
control signal for the objective lens if the integrated value falls outside a preset
reference range.
24. The optical disc device of claim 22, wherein the second means calculates a mean value
of the detected low frequency component values, and generates the tilt servo control
signal for the objective lens if the mean value falls outside a preset reference range.